A known image forming apparatus is configured such that a process cartridge having a photosensitive member is detachably attachable to a casing. The image forming apparatus is provided with first and second guide grooves that guide first and second guided portions, which are spaced apart from each other in the process cartridge, respectively.
The first and second guide grooves each have an upstream-side guide surface and a downstream-side guide surface in a process cartridge attaching direction. The upstream-side guide surface extends toward a downstream side in the process cartridge attaching direction, at an inclination from upward to downward. The downstream-side guide surface extends from the upstream-side guide surface further toward the downstream side in the attaching direction at an inclination closer to the horizontal than that of the upstream-side guide surface. The second guide groove has a mounting portion extending further downward from the downstream-side guide surface. The first guide groove and the second guide groove define the position of the process cartridge being attached or removed.